(1) Field of the Invention
The invention relates to the general field of photoresist processing during integrated circuit manufacturing, more particularly to automatic dispensing of photoresist developer.
(2) Description of the Prior Art
In the course of manufacturing semiconductor integrated circuits, extensive use is made of photoresist for masking purposes. The photoresist, after exposure to ultraviolet light through a mask, needs to be developed. In a production environment there will be multiple stations at which photoresist can be developed as well as automatic delivery systems for supplying and dispensing developer solution, as it is needed.
We have observed, with the photoresist developer supply system that we have been using, that certain defects in the developed image may manifest themselves from time to time. Careful examination has revealed that these defects followed the direction of developer solution flow along the wafer surface from its point of dispensation. Further examination has shown that the defects arise from microbubbles in the developer solution. These microbubbles are introduced into the solution during its journey from the chemical room, where the developer is stored in bulk, to the buffer tanks in which the developer is held in readiness for demand by the dispensing stations.
Referring now to FIG. 1, we show there a schematic view of the flow system that we had previously used to bring the developing solution to the wafers. Main entrance pipe 3 is connected to the chemical room (not shown) through several tens of meters of piping. From 3, photoresist developer is directed to buffer tanks 1 and 2 whence they flow to two-way valve 4 which, under external control, directs the liquid to either of its outputs 5 and 6, each of which is connected to a separate dispensing unit. The symbol `P in a circle` represents a diaphragm pump.
The sequence of events whereby alternate buffers are used as sources of photoresist developer is best followed by referring to FIGS. 2 and 3. Block 23 in FIG. 2 is marked as `active` meaning that it is in process of sending liquid to two-way valve 4 (FIG. 1) The time for which it is active corresponds to the width of block 23 and is about 40 minutes. In synchrony with this, we have blocks 24 and 25 that relate to buffer 2. During the time corresponding to block 24, buffer tank 2 is connected to exhaust tank 13 through two-way valve 12. This brings the pressure in buffer 2 to zero (relative to atmospheric pressure) and allows developer solution to flow in from 3 and refill 2. When level sensor 32 signals that buffer 2 is full, valve 12 switches over to a source of inert gas (usually nitrogen) that is maintained at positive pressure (relative to atmosphere) with a view to minimizing possible oxidation of the developer in tank 2. Said positive pressure is then maintained for the period corresponding to block 25 when buffer 2 is said to be in standby mode.
Thus, except for the brief period corresponding to block 24, buffer 2 is always under a positive pressure of about 1 kg/cm..sup.2 This is reflected in FIG. 3 which shows pressure vs. time for the two tanks. During the time that buffer 1 is active (block 23 in FIG. 2) the pressure in it is about 1 kg/cm..sup.2 (curve 33 in FIG. 3). While buffer 2 is refilling (block 24 in FIG. 2) the pressure is reduced to zero (relative to atmospheric) (curve 36) but, once the tank has been filled, the pressure reverts to 1 kg/cm..sup.2 (curve 37) and remains there until it needs to be refilled once more (curve 38 and block 28).
A similar sequence of events, displaced in time so as to be exactly out of phase with tank 2's sequence, takes place for tank 1 (blocks 23, 26, and 27 in correspondence with curves 33, 34, and 35 respectively). Thus, one tank is always active while the other is being filled and then placed on standby status. In all cases, however, except for the brief period of refilling, the pressure in the tanks is positive and it is never negative.